Glycoprotein (GP) IIb-IIIa serves as the platelet fibrinogen receptor. Studies of the tertiary structure of GPIIIa have shown that the protein has a large loop structure of at least 325 amino acids in length. To further characterize this loop structure, intact platelets were digested with alpha-chymotrypsin. Digestion products were examined using the anti-GPIIIa monoclonal antibodies (MoAbs) AP3, D3GP3, and C5GP3, as well as the human alloantibody, anti-PLA1. AP3 recognized GPIIIa digestion products of 109, 95, and 68 Kd. D3GP3 and C5GP3 recognized an additional band of 51 Kd. Time course digestions demonstrated that the 51-Kd fragment was generated by proteolysis of the 68-Kd peptide. Sequence analysis of the reduced 51-Kd peptide showed that this fragment began at amino acid 422. The nonreduced 51-Kd peptide was reactive with antibodies directed against the first 13 amino acids of GPIIIa, demonstrating the presence of a covalently attached N-terminal peptide. These data suggest that: (1) the minimum length of the loop structure is at least 384 amino acids; (2) the AP3 epitope is formed at least in part by a determinant contained within residues 348 to 421; and (3) the D3GP3 and C5GP3 epitopes are contained within amino acids 422 to 692 of GPIIIa, a region that may be flexible and involved in conformational changes that occur after ligand binding.